This invention relates to asynchronous data communications and in particular to a device designed for sending ATM or like cells over transmission links, for example T1 or E1 trunks.
Reference is made throughout this specification to cells. These cells are typically ATM cells, which are small packets of fixed size having a payload of 48 bytes and a header of five bytes.
In order to meet the increasing bandwidth demands of ATM (Asynchronous Transfer Mode) a technique is known that combines a number of slower TDM links, such as T1 and E1 links, into an aggregate channel of higher bandwidth. A technique for doing this is known as Inverse Multiplexing for ATM (IMA) and is described in the ATM Forum standard AF-PHY-0086.000.
ATM cells are subject to variable delays over the different links, and a protocol must be established to reconstruct the original cell stream at the receiving end. In accordance with the ATM Forum standard, an IMA Frame is defined and special cells, defined as IMA Control Protocol cells (ICP), are transmitted once per IMA frame.
When a transmitter is preparing to send ATM cells over an IMA channel, it must identify a group of N physical links that are to make up the channel. It then assigns a Link Identifier (LID) to each link and transmits this to the far end in ICP cells. This is a logical link identifier between 0 and 31, which is different from the physical identity of the link. The ATM cells are then transmitted over the identified links in a cyclic fashion in ascending order based on the LID assigned to each physical link. The receiver assembles the incoming cells according to the ascending order based on the link ID.
The IMA protocol defines an IMA frame that is defined as M consecutive cells on each link. The IMA frames are transmitted simultaneously on each link. An ICP cell is sent on each link once per IMA frame. An offset number identifies the position of the ICP cell, which can be different for each link. If there are no ATM layer cells to be sent in an IMA frame, filler cells are transmitted to maintain a continuous stream of cells at the physical layer.
While the need for aggregate bandwidth is sometimes present, that is not always the case, and sometimes it is desirable to set up a standard UNI (User Network Interface) mode on each link.
An object of the invention is an efficient and cost-effective device capable of providing both an IMA and a UNI (User Network Interface) protocol interface.
According to the present invention a termination device is a termination device for connection to a group of TDM (time division multiplexed) links comprising an interface circuit for receiving an incoming cell stream forming one or more virtual channels; a buffer memory for storing incoming cells at memory locations identified by pointers; a pointer queue for each virtual channel, said pointer queue indicating the memory locations of queued cells in each channel; a control unit for interrogating the pointer queue and assigning the next incoming cell to the next free memory location for the associated channel; a scheduler selectively operable to sequentially assign cells from each queue in said buffer memory either to individual links or to a group of links forming an inverse multiplexed group; and a per link output circuit for placing cells from said memory locations on said links to form outgoing virtual channels.
The interface circuit is preferably provides a Utopia level 2 interface for connection to an ATM adaptation layer device. The links are physical links in the PDH (Plesiochronous Digital Hierarchy) network transporting the data, for example T1 or E1 trunks. Depending upon whether the device is in UNI mode or IMA mode, a channel will consist of a single link or a group of physical links having an aggregate bandwidth equal to the sum of the bandwidth of the individual links. In IMA mode, the round robin scheduler distributes successive ATM cells as determined by their address pointers to successive physical links in ascending order based on their link ID numbers. In UNI mode, the successive ATM cells are passed directly to the outgoing link associated with their channel.
The input circuit preferably includes filtering functions. An address is assigned to each channel, which can either be an IMA channel consisting of a group of physical links or a UNI channel consisting of a single link. There can be eight UNI links and up to 4 IMA groups, so that 12 Utopia addresses are provided.
The invention also provides a termination device for connection to a group of TDM (time division multiplexed) links and capable of receiving streams of cells forming a virtual channel over one or more of said trunks, said cell streams arriving on said individual links or on a plurality of said links forming an inverse multiplexed (IMA) group, comprising a per link input circuit for receiving incoming cell streams on said links; a per link cell delineation circuit for delineating incoming cells; a per link means for segregating control cells from user cells; a per group rate recovery block to generate the IDCR (IMA Data Cell Rate); a buffer memory for storing incoming cells of an IMA group at specific memory locations; a per group inverse scheduler controlled by the IDCR and for arranging incoming cells of an IMA group in the correct order; an output circuit for outputting cells to a receiving device through an interface; and a memory controller selectively operable to direct incoming cells to said memory in an inverse multiplexing mode or to said output circuit in a UNI mode.